Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The compound is often encountered as one of its hydrates, FeO(OH)·nH 2O [rust]. The monohydrate FeO(OH)·H 2O is often referred to as iron(III) hydroxideFe(OH) 3,hydrated iron oxide, yellow iron oxide, or Pigment Yellow 42.
Anhydrous ferric hydroxide occurs in the nature as the exceedingly rare mineral bernalite, Fe(OH)3·nH2O (n=0.0-0.25). Iron oxyhydroxides, FeOOH, are much more common and occur naturally as structurally different minerals (polymorphs) denoted by the Greek letters α, β, γ and δ.
Goethite, α-FeO(OH), has been used as an ochre pigment since prehistoric times.
Akaganeite is the β polymorph, formed by weathering and noted for its presence in some meteorites and the lunar surface. However, recently it has been determined that it must contain some chloride ions to stabilize its structure, so that its more accurate formula is FeO 0.833(OH) 1.167Cl 0.167 or Fe 6O 5(OH) 7Cl.
Lepidocrocite, the γ polymorph, is commonly encountered as rust on the inside of steel water pipes and tanks.
Feroxyhyte (δ) is formed under the high pressure conditions of sea and ocean floors, being thermodynamically unstable with respect to the α polymorph (goethite) at surface conditions.
The color of iron(III) oxyhydroxide ranges from yellow through dark-brown to black, depending on the degree of hydration, particle size and shape, and crystal structure.
The crystal structure of β-FeOOH (akaganeite) is that of hollandite or BaMn 8O 16. The unit cell is tetragonal with a=1.048 and c=0.3023 nm, and contains eight formula units of FeOOH. Its dimensions are about 500 × 50 × 50 nm. Twinning often produces particles with the shape of hexagonal stars. 
On heating, β-FeOOH decomposes and recrystallizes as α-Fe 2O 3 (hematite).
Limonite, a mixture of various hydrates and polymorphs of ferric oxyhydroxide, is one of the three major iron ores, having been used since at least 2500 BC.
^ abA. L. Mackay (1962): "β-Ferric oxyhydroxide—akaganéite", Mineralogical Magazine (Journal of the Mineralogical Society), volume 33, issue 259, pages 270-280 doi:10.1180/minmag.1962.033.259.02
^C. Rémazeilles and Ph. Refait (2007): "On the formation of β-FeOOH (akaganéite) in chloride-containing environments". Corrosion Science, volume 49, issue 2, pages 844-857. doi:10.1016/j.corsci.2006.06.003
^MacEachern, Scott (1996): "Iron Age beginnings north of the Mandara Mountains, Cameroon and Nigeria". InIn Pwiti, Gilbert and Soper, Robert (editors) (1996) Aspects of African Archaeology: Proceedings of the Tenth Pan-African Congress University of Zimbabwe Press, Harare, Zimbabwe, ISBN 978-0-908307-55-5, pages 489-496. Archived here on 2012-03-11.
^Diop-Maes, Louise Marie (1996): "La question de l'Âge du fer en Afrique" ("The question of the Iron Age in Africa"). Ankh, volume4/5, pages 278-303. Archived on 2008-01-25.
^Safoora Rahimi, Rozita M. Moattari, Laleh Rajabi, Ali Ashraf Derakhshan, and Mohammad Keyhani (2015): "Iron oxide/hydroxide (α,γ-FeOOH) nanoparticles as high potential adsorbents for lead removal from polluted aquatic media". Journal of Industrial and Engineering Chemistry, volume 23, pages 33-43. doi:10.1016/j.jiec.2014.07.039
^ abTim Grundl and Jim Delwiche (1993): "Kinetics of ferric oxyhydroxide precipitation". Journal of Contaminant Hydrology, volume 14, issue 1, pages 71-87. doi:10.1016/0169-7722(93)90042-Q
^K. H. Gayer and Leo Woontner (1956): "The Solubility of Ferrous Hydroxide and Ferric Hydroxide in Acidic and Basic Media at 25°". Journal of Physical Chemistry, volume 60, issue 11, pages 1569–1571. doi:10.1021/j150545a021
^ abcEgon Matijević and Paul Scheiner (1978): "Ferric hydrous oxide sols: III. Preparation of uniform particles by hydrolysis of Fe(III)-chloride, -nitrate, and -perchlorate solutions". Journal of Colloid and Interface Science, volume 63, issue 3, pages 509-524. doi:10.1016/S0021-9797(78)80011-3
^Dan Li, Xiaohui Wang, Gang Xiong, Lude Lu, Xujie Yang and Xin Wang (1997): "A novel technique to prepare ultrafine Fe 2O 3 via hydrated iron(III) nitrate". Journal of Materials Science Letters volume 16, pages 493–495 doi:10.1023/A:1018528713566
Whittemore and Donald Langmuir (1974): "Ferric Oxyhydroxide Microparticles in Water". Environmental Health Perspective, volume 9, pages 173-176. doi:10.1289/ehp.749173